Digital representation of audio waveforms using peak shifting to provide increased dynamic range

ABSTRACT

A system allows greater dynamic range in fixed-width sample representations of waveforms. Known properties of the waveform are used to determine an unlikely, or impossible, data condition to be used as an exception condition. Samples following the exception condition are assumed to be shifted by a predetermined amount so that their representation within the allowable dynamic range of the word results in an extended dynamic range. In a preferred embodiment, pre-processing of a waveform is used to create exception conditions and to shift portions of the waveform that would otherwise become clipped, so that the waveform peaks reside in a shifted position within the digital representation. Analogously, playback processing serves to detect the exception conditions and to restore the peaks of the waveform back to their original status for, e.g., audible digital-to-analog playback, high-resolution processing, etc.

BACKGROUND OF THE INVENTION

This invention relates in general to digital representation of data andmore specifically to a system for increasing the range of representablevalues in waveforms.

Digital processing is used advantageously in many aspects of signalprocessing including audio, image, radio-frequency, infrared, mathfunctions, etc. A signal is typically captured to the digital domain byanalog-to-digital conversion of a physical signal to a digital waveformrepresentation. Once the physical signal has been captured digitally, itcan be modified, manipulated, copied, transferred, etc., with little orno degradation to the waveform and with many advantages provided bydigital processing.

However, a problem arises with traditional approaches to digitalwaveform representation. Typically, the waveform is represented by alarge number of “samples.” Each sample is a digital value (usuallybinary) and is represented by a digital “word” that is a fixed number ofbits in width. The number of bits in a word dictates the possible rangeof values that can be represented in a sample corresponding to the word.If the sample needs a value that is larger than can be represented by aword then the waveform can not be accurately represented and a conditionknown as “clipping” occurs.

When an audio signal is digitally clipped, the waveform representationno longer is a true representation of the desired audio signal. Instead,the sample values are represented by the closest value allowed by theword width in a processing system. Typically, this means that the wordvalues are at their highest maximum positive or negative magnitudes forthe duration of the signal extending beyond the representable values ofthe sample size. The range of values that can be handled by a processingsystem is referred to as the “dynamic range” or “bit width,” “wordwidth,” etc., of the system.

Since different systems may have different dynamic ranges, the clippingeffect may make it undesirable, or impossible, for different systems towork together or to exchange data. Increasing the dynamic range of asystem is often not possible after the system has been manufactured. Theproduction of systems with larger dynamic range is more expensive andoften more difficult since more processing power, larger buses, higherperformance and wider memory, and improvements in other resources mustbe included in the systems.

SUMMARY OF THE INVENTION

The present invention provides a mechanism allowing greater dynamicrange in fixed-width sample representations of waveforms. Knownproperties of the waveform are used to determine an unlikely, orimpossible, data condition to be used as an exception condition. In apreferred embodiment that is directed to processing of audio waveforms,a first large value transitioning to a second large value of differentsign is used to indicate an exception condition. Samples following theexception condition are assumed to be shifted by a predetermined amountso that their representation within the allowable dynamic range of theword results in an extended dynamic range.

In a preferred embodiment, pre-processing of a waveform is used tocreate exception conditions and to perform “peak packing” to shiftportions of the waveform that would otherwise become clipped, so thatthe waveform peaks reside in a shifted position within the digitalrepresentation. Analogously, playback processing serves to detect theexception conditions and to restore the peaks of the waveform back totheir original status for, e.g., audible digital-to-analog playback,high-resolution processing, etc.

In one embodiment A method for processing a digitally representedwaveform, wherein the digitally represented waveform includes multiplewords of a fixed bit-width, wherein each word has a value, the methodcomprising determining when a word's value exceeds a predeterminedthreshold; shifting the value of the word to result in a value that doesnot exceed the predetermined threshold; and replacing the word's valuewith the shifted value.

In another embodiment the invention provides a method for reconstructinga digitally represented waveform, wherein the digitally representedwaveform includes multiple words of a fixed bit-width, wherein each wordhas a value, wherein a predefined exception condition is used toindicate special processing of one or more of the multiple words, themethod comprising detecting the exception condition; and

upon detection of the exception condition performing an operation tomodify one or more subsequent word values.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a prior art illustration of a waveform representation;

FIG. 1B illustrates a peak packing operation according to the presentinvention;

FIG. 1C shows the result of unpacking the packed waveform of FIG. 1B;

FIG. 2A shows two iterations of peak packing;

FIG. 2B illustrates unpacking of the waveform values of FIG. 2A;

FIG. 3A illustrates a waveform with both positive and negative peakpacking;

FIG. 3B shows the reconstruction of the packed waveform of FIG. 3A;

FIG. 4 illustrates details of the invention at the word value level;

FIG. 5A shows a flowchart of basic steps of a packing routine; and

FIG. 5B shows a flowchart of basic steps of an unpacking routine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A is a prior art illustration of a waveform representation.

In FIG. 1A, waveform 10 is represented by fixed-width words havingdynamic range 20. For example, if the words are 8 bits in width then thedynamic range is 256 values. If the word width is 16 bits then thedynamic range is 64k values. If the word 30 width is 32 bits then thedynamic range is 2^32 values, etc. Note that any suitable word width canbe used.

Waveform 10 includes a clipped region 30 where the desired waveformvalues exceed the dynamic range of the words. This results in a loss ofthe signal data shown as a dashed line at 40. Rather than provide anaccurate reproduction of the signal, all signal information above themaximum representable value at 50 is lost. Instead, the waveform isclipped to a flat region that stays at the closest possiblerepresentable value. Note that such clipping can also occur in anegative portion of the waveform if a signal extends below the maximumnegative magnitude at 52. A discussion of the approach of the inventionwith respect to positive peaks is also applicable to negative peaks.

Note that any form of word number or value representation is possible.In other words, the word values can be used in any digital numberingscheme (e.g., binary, octal, hexadecimal, etc.) in unsigned integer,sign-magnitude, one's complement, two's complement, floating point orany suitable format. For case of discussion, a signed integer valuerepresentation is assumed.

As is known in the art, the waveform changes with time. Time isrepresented as increasing to the right along the horizontal axis. Thecentral axis, midway between maximum 50 and minimum 52, represents azero value.

FIG. 1B illustrates an approach of the present invention to representthe otherwise clipped peak within the same dynamic range as thatdepicted in FIG. 1A.

In FIG. 1B, waveform 100 corresponds to the same signal discussed inFIG. 1A. Note that peak 140 that is clipped in FIG. 1A is now preservedby shifting the values corresponding to the peak to start at minimum152. In this manner, the waveform can be represented by an effectivelylarger dynamic range although the word width does not need to vary.Vertical lines 160 and 162 are not actually part of the waveformrepresentation but are used to show a discontinuity in the waveformrepresentation. In a preferred embodiment, these discontinuities areused to indicate an exception condition to alert a playback processingdevice to reposition the shifted peak data to achieve the originalsignal.

FIG. 1C shows the result of repositioning the shifted peak data of FIG.1B. In FIG. 1C, peak 140 of FIG. 1B is shown restored to the waveform toachieve an accurate reproduction of the signal. This approach can beused, for example, to obtain higher quality digital-to-analog (D/A)conversion of signals with a given word width. Also, where differentprocessing systems use different word widths it is possible to process asignal from a larger word width system in a smaller word width system.Transfer of signals over bus widths that are smaller than the originalwaveform word width representation are possible.

The exception condition is chosen so that it is impossible, or unlikely,to occur with a valid waveform. For a real-world audio signal, the eventof waveform values swinging from the largest positive to the largestnegative value in adjacent samples will not occur. Neither will valuesswing from largest negative to largest positive values. In otherembodiments, other exception conditions can be used depending onpredictable properties or characteristics of the waveform. For example,smaller swings, second or third derivatives, impulse patterns, value orbitwise anomalies, etc., can all be used to indicate an exceptioncondition.

In the preferred embodiment, the exception condition uses ranges ofvalues, rather than absolute maximum (i.e., largest positive) andabsolute minimum (i.e., largest negative) values. For example, anexception condition is defined as where a first word value is above afirst threshold near the maximum and the next word value is below asecond threshold near the minimum. The thresholds are used to setmaximum and minimum ranges where, if two adjacent word values lie ineach of the ranges, an exception condition occurs. Other types ofexception conditions can require multiple words to be within one or morespecific ranges, require multiple alternations among ranges or values,etc. In general, any type of exception condition can be used.

Note that the approach of a preferred embodiment uses the same number ofwords to represent a waveform with increased dynamic range. Thisapproach is referred to as “peak packing.” A peak packing process can beperformed quickly, such as in real time, because it only requirescomparing values of two adjacent words. A clipping condition is known bycomparing a desired word value to the limits of the dynamic range of atarget data stream. If the desired value exceeds the dynamic range thenpeak packing is performed. In a preferred embodiment, the modificationto word values in the clipping region is a simple shift of the values.Other embodiments can use other types of processing such as comparison,combination or other detection of multiple word values and more complexlogical or arithmetic operations for processing word values in theclipping region. Similarly, reconstruction of a peak packed waveformrequires only small amounts of processing and can be done on-the-fly todetect the exception condition and reverse the shifting (or otherprocessing) operation.

In general, any type of processing to deconstruct a signal to a peakpacked waveform and to reconstruct a signal from a peak packed waveformis possible and within the scope of the invention. For example, theprocessing can be done in real time or non-real time. The processing canbe performed in hardware or software or by a combination of both.Different types of processing (e.g., using different exceptionconditions) can be performed at different times, and in differentregions of a waveform. It may be desirable to perform peak packing onportions of a waveform that are not being clipped (i.e., that remainwithin the dynamic range of the word width). Other advantages willbecome apparent.

FIGS. 2A and 2B illustrate multiple peak packing.

FIG. 2A, a signal has been peak packed into a fixed dynamic range by twoiterations of peak packing. A fist clip condition occurs at 200 andsubsequent waveform values are shifted to the opposite extent of theword value representations (i.e., maximum negative). At 210, the peakpacking operation still results in a clip condition so another shift isperformed so that subsequent words of peak 220 are now subjected to thecompounded operation of two shifts (or the equivalent). When peak 220'svalues fall at or near the greatest negative extent of the dynamicrange, the word values are no longer subjected to the second shiftoperation so that the values resume at 230 subjected only to the firstshift. As the waveform values continue to decrease in value theyeventually again approach the greatest negative extent whereupon theyare no longer subjected to the first shift operation and so resume attheir normal representation at 240.

FIG. 2B shows the reconstruction of the peak packed signal of FIG. 2A.As can be seen by the vertical (i.e., time) correspondence between theFIGS. , the peak segments between 200 and 240 are shifted by multiplesof the dynamic range to achieve an accurate reproduction of a signalthat extends well beyond the standard word value representation.

FIG. 3A illustrates a waveform with both positive and negative peakpacking. FIG. 3B shows the reconstruction of the packed waveform of FIG.3A. Note that any number of waveform cycles and any shape or type ofwaveform can be used with the approach of the present invention.

FIG. 4 illustrates details of an approach of the invention.

In FIG. 4, waveform 254 is shown with portions expanded so that valuesare represented as points in grid sections. Grid section 260 includescolumns corresponding to word values at 270, 272, 274, etc. Each wordhas a numeric value that is represented by a solid square at thecorresponding waveform point position. For example, assuming a 16-bittwo's complement representation, the value of words 270 and 272 is2^15−1−9=32758. The value of word 274 is 2^15−1−1=32766. This type ofrepresentation is well-known in the art.

Word 276 does not have sufficient dynamic range to represent its signalvalue which would be 32768. Thus, the technique of the present inventionshifts the value by 16 bits, (or, analogously, divides or subtracts, orperforms other functions or combinations of functions) to generate a newvalue for word 276 of −32768. This results in a very large differencebetween adjacent words 274 and 276 to produce a discontinuity indicatedby the arrow at 280. The transition is from a large positive number,32766, to a large negative number, −32768. This transition is easilydetectable upon playback or other processing of the waveform and will beused as an exception condition.

Note that an exception condition occurs in the other direction, at 282,from word 286 to word 288, to produce a detectable exception conditionto restore the normal interpretation (i.e., unshifted) of the values ofthe waveform.

A preferred embodiment detects when adjacent word values transition froma first range to a second range. In the example of FIG. 4, the firstrange is indicated at 290 and includes the values 32764 through 32767.The value 32766 is considered a “threshold” value so that any valuesabove it are considered to be within the first range. A second range isdefined at 292 to include values of −32768 through −32764. Naturally,any range of values can be used. Multiple different ranges, regions,area, sets, etc. of values can be used to define exception conditions.The detection of an exception condition does not have to be betweenadjacent words but can use separated words, groups of words, mere areasor portions of words, etc.

FIG. 5A illustrates a flowchart of basic steps in a routine to performthe packing operation according to one aspect of the invention.

In FIG. 5A, the packing routine is entered at 300 where it is assumedthere is a waveform represented by input values to be packed into astream of words. At step 302, a shift count is set to zero. A zero valuefor the shift count means no shift will be performed. A positive valuefor the shift count indicates large positive values are to be shifteddownward, or decreased in value, so that they will fit within thedynamic range of the word stream.

At step 304 a next input value from a waveform to be packed is obtained.Note that the input values can be obtained from memory, an input port,processor, network, or any other source. The input values can be derivedfrom physical signals via an analog-to-digital converter (A/D) or otherdevice. At step 306 any designated shifting is performed to the inputvalue.

Next, a check is made at step 308 as to whether the input value exceedsthe dynamic range. If not, the value is stored into a word at step 310and execution proceeds to step 304 to get successive values. Otherwiseif, at step 308, the input value exceeds the dynamic range, step 312 isperformed to modify the shift count dependent on whether the excessivevalue is greater than, or less than, the values that can be representedby the dynamic range of the target word width. For example, if the inputvalue is greater than the maximum allowable for the dynamic range thenthe shift count is incremented. If the input value is less than theminimum allowable then the shift count is decremented.

Execution continues until there is no more input data or until someother condition (e.g., user control) causes termination.

FIG. 5B illustrates basic steps in a routine to perform unpacking.

In FIG. 5B, an unpacking routine is entered at 320 when it is desired tounpack a packed waveform to obtain the original signal. At step 322 theshift count is set to 0 and the first word of the waveform is obtained.At step 324 the shift count is used to apply shifting, if needed. Asbefore, a shift count of zero indicates that no shift is to be performedand the word value is used to generate a signal amplitude, or point. Ifthe shift count is positive then the word value is increased by shiftingup (i.e., left) by the number of bits in the word width multiplied bythe shift count. Similarly, if the shift count is negative then the wordvalue is shifted down by the number of bits in the word width multipliedby the absolute value of the shift count.

The shifted value is stored, or used, as a point in the reconstructedsignal. For example, the point can be stored in the traditional mannerin a word of sufficient width. The point can also be translated, e.g.,to a voltage, current or other physical property, so that the signal canbe played back, used as a control signal, etc. The original word valueis stored so that it can be used in the comparison at step 328,described, below.

The above discussion with respect to steps 320-324 assumes that thefirst word of a waveform is not shifted. If it is necessary to shift thefirst word value then the shift count can be supplied by an externalroutine or other source and it will not be initialized at step 322 whenthe unpacking routine is entered.

At step 326, the next word is obtained. Step 328 checks whether the lasttwo word values obtained are over different thresholds. In other words,if there is a large (i.e., greater than threshold-to-threshold) swing ofadjacent word values this exception condition is detected by step 328.If an exception condition is not detected then processing returns tostep 324 where any necessary shifting is performed on the current wordvalue (i.e., the one just obtained at step 326). If an exceptioncondition is detected then step 330 is executed to modify the shiftcount in accordance with the direction of the exception condition. Inother words, if the swing is from positive to negative (or high to low,greater to lesser, etc.) then the shift count is incremented. If theswing is from negative to positive the shift count is decremented.

Many other ways of performing packing and unpacking will be apparent.The routines depicted in this application are merely basic examples.Steps can be performed by hardware or software, as desired. Note thatsteps can be added to, taken from or modified from the steps in theflowcharts presented in this specification without deviating from thescope of the invention. In general, the flowcharts are only used toindicate one possible sequence of basic operations to achieve afunctional aspect of the present invention.

Any suitable programming language can be used to implement the routinesof the present invention including C, C++, Java, assembly language, etc.Different programming techniques can be employed such as procedural orobject oriented. The routines can execute on a single processing deviceor multiple processors. Although the flowchart format demands that thesteps be presented in a specific order, this order may be changed.Multiple steps can be performed at the same time. The flowchart sequencecan be interrupted. The routines can operate in an operating systemenvironment or as stand-alone routines occupying all, or a substantialpart, of the system processing.

Although the invention has been described with respect to specificembodiments, thereof, these embodiments are merely illustrative, and notrestrictive of the invention. For example, although the invention isdirected to audio applications, other types of waveform representationscan be used such as image, radar, mathematical functions, etc. Ingeneral, any waveform with at least one predictable characteristic orproperty from which an exception condition can be detected can besuitable for use with the present invention.

Note that exception conditions can be used for other purposes ratherthan peak packing. For example, the number, type, pattern or otherproperties of one or more packed or otherwise modified waveforms can beused to, e.g., encrypt or encode a signal, place a signature orwatermark onto the signal, impose text or numerical information onto thewaveform, etc.

Thus, the scope of the invention is to be determined solely by theappended claims.

1. A method for creating a digital representation of a waveform, whereinthe digital representation includes multiple words each having a valueso that possible values define a range of representation, the methodcomprising: using a value of a first word in the digital representationto create a discontinuity in the digital representation of the waveform;inserting subsequent words to represent a portion of the digitalwaveform continuously relative to the first word; and outputting atleast a portion of the words and the discontinuity so that the range ofrepresentation of the digital waveform can be increased duringpresentation of the waveform in response to detecting the discontinuity.2. The method of claim 1, wherein the waveform is an audio waveform,wherein the presentation includes an audio presentation.
 3. The methodof claim 1, wherein the waveform is an image waveform, wherein thepresentation includes an image presentation.
 4. The method of claim 1,wherein the waveform is a math function, wherein the presentationincludes visually depicting at least a portion of the math function. 5.The method of claim 2, wherein the word values are time sequenced, themethod further comprising: detecting when a first word value is near amaximum word value; and subsequently modifying a next word value in thetime sequence to be near a minimum word value.
 6. The method of claim 1,wherein a word value can be positive or negative.
 7. The method of claim1, wherein a word value is an unsigned integer value.
 8. A method forpresenting a digitally represented waveform, wherein the digitallyrepresented waveform includes multiple words, wherein each word has avalue so that possible values define a range of representation, themethod comprising: detecting a first discontinuity at a first word inthe digitally represented waveform; detecting words subsequent to thefirst word; detecting a second discontinuity at a second word in thedigitally represented waveform; and adjusting values of at least aportion of the subsequent words between the first and second words sothat a continuous waveform having an increased range of representationis presented.
 9. The method of claim 8, further comprising: determiningthat a first word value is above a first predetermined threshold; anddetermining that a second word value is below a second predeterminedthreshold.
 10. The method of claim 8, further comprising: shifting theone or more subsequent values to restore a portion of the digitallyrepresented waveform.
 11. The method of claim 8, wherein an exceptioncondition is determined by comparing a transition to the first wordvalue from a word value prior to the first word.
 12. The method of claim11, wherein the exception condition is a transition from a large valueto a small value.
 13. The method of claim 12, wherein the exceptioncondition is a transition from a maximum value to a minimum value. 14.The method of claim 11, wherein the exception condition includes aderivative of a change in multiple word values.
 15. The method of claim11, wherein the exception condition includes a bit pattern anomaly in aword value.
 16. The method of claim 11, wherein the exception conditionincludes a first word value in a first range and a second word value ina second range.
 17. An apparatus for creating a digital representationof a waveform, wherein the digital representation includes multiplewords each having a value so that possible values define a range ofrepresentation, the apparatus comprising: a processor; amachine-readable storage medium including instructions executable by theprocessor for using a value of a first word in the digitalrepresentation to create a discontinuity in the digital representationof the waveform; inserting subsequent words to represent a portion ofthe digital waveform continuously relative to the first word; andoutputting at least a portion of the words and the discontinuity so thatthe range of representation of the digital waveform can be increasedduring presentation of the waveform in response to detecting thediscontinuity.
 18. A computer-readable storage medium includinginstructions executable by a processor for creating a digitalrepresentation of a waveform, wherein the digital representationincludes multiple words each having a value so that possible valuesdefine a range of representation, the computer-readable storage mediumcomprising: using a value of a first word in the digital representationto create a discontinuity in the digital representation of the waveform;inserting subsequent words to represent a portion of the digitalwaveform continuously relative to the first word; and outputting atleast a portion of the words and the discontinuity so that the range ofrepresentation of the digital waveform can be increased duringpresentation of the waveform in response to detecting the discontinuity.19. An apparatus for presenting a digitally represented waveform,wherein the digitally represented waveform includes multiple words,wherein each word has a value so that possible values define a range ofrepresentation, the apparatus comprising: a processor; amachine-readable medium including one or more instructions executable bya processor for detecting a first discontinuity at a first word in thedigitally represented waveform; detecting words subsequent to the firstword; detecting a second discontinuity at a second word in the digitallyrepresented waveform; and adjusting values of at least a portion of thesubsequent words between the first and second words so that a continuouswaveform having an increased range of representation is presented.
 20. Acomputer-readable storage medium for presenting a digitally representedwaveform, wherein the digitally represented waveform includes multiplewords, wherein each word has a value, the computer-readable storagemedium comprising one or more instructions for: detecting a firstdiscontinuity at a first word in the digitally represented waveform;detecting words subsequent to the first word; detecting a seconddiscontinuity at a second word in the digitally represented waveform;and adjusting values of at least a portion of the subsequent wordsbetween the first and second words so that a continuous waveform ispresented.